5.4 Logical Architecture

03.603GPPGeneral Packet Radio Service (GPRS)Release 1998Service descriptionStage 2TS

GPRS is logically implemented on the GSM structure through the addition of two network nodes, the Serving GPRS Support Node and the Gateway GPRS Support Node. It is necessary to name a number of new interfaces. No inference should be drawn about the physical configuration on an interface from Figure 2.

Figure 2: Overview of the GPRS Logical Architecture

5.4.1 GPRS Support Nodes

A GPRS Support Node (GSN) contains functionality required to support GPRS. In one PLMN, there may be more than one GSN.

The Gateway GPRS Support Node (GGSN) is the node that is accessed by the packet data network due to evaluation of the PDP address. It contains routeing information for attached GPRS users. The routeing information is used to tunnel N‑PDUs to the MS’s current point of attachment, i.e., the Serving GPRS Support Node. The GGSN may request location information from the HLR via the optional Gc interface. The GGSN is the first point of PDN interconnection with a GSM PLMN supporting GPRS (i.e., the Gi reference point is supported by the GGSN).

The Serving GPRS Support Node (SGSN) is the node that is serving the MS (i.e., the Gb interface is supported by the SGSN). At GPRS attach, the SGSN establishes a mobility management context containing information pertaining to e.g., mobility and security for the MS. At PDP Context Activation, the SGSN establishes a PDP context, to be used for routeing purposes, with the GGSN that the GPRS subscriber will be using.

The SGSN and GGSN functionalities may be combined in the same physical node, or they may reside in different physical nodes. SGSN and GGSN contain IP routeing functionality, and they may be interconnected with IP routers. When SGSN and GGSN are in different PLMNs, they are interconnected via the Gp interface. The Gp interface provides the functionality of the Gn interface, plus security functionality required for inter-PLMN communication. The security functionality is based on mutual agreements between operators.

The SGSN may send location information to the MSC/VLR via the optional Gs interface. The SGSN may receive paging requests from the MSC/VLR via the Gs interface.

5.4.2 GPRS Backbone Networks

There are two kinds of GPRS backbone networks. These are called:

– intra-PLMN backbone network; and

– inter-PLMN backbone network.

The intra-PLMN backbone network is the IP network interconnecting GSNs within the same PLMN.

The inter-PLMN backbone network is the IP network interconnecting GSNs and intra-PLMN backbone networks in different PLMNs.

Figure 3: Intra- and Inter-PLMN Backbone Networks

Every intra-PLMN backbone network is a private IP network intended for GPRS data and GPRS signalling only. A private IP network is an IP network to which some access control mechanism is applied in order to achieve a required level of security. Two intra-PLMN backbone networks are connected via the Gp interface using Border Gateways (BGs) and an inter-PLMN backbone network. The inter-PLMN backbone network is selected by a roaming agreement that includes the BG security functionality. The BG is not defined within the scope of GPRS. The inter-PLMN backbone can be a Packet Data Network, e.g., the public Internet or a leased line.

5.4.3 HLR

The HLR contains GPRS subscription data and routeing information. The HLR is accessible from the SGSN via the Gr interface and from the GGSN via the Gc interface. For roaming MSs, HLR may be in a different PLMN than the current SGSN.

5.4.4 SMS-GMSC and SMS-IWMSC

The SMS-GMSC and SMS-IWMSC are connected to the SGSN via the Gd interface to enable GPRS MSs to send and receive SMs over GPRS radio channels.

5.4.5 GPRS Mobile Stations

A GPRS MS can operate in one of three modes of operation. The mode of operation depends on the services that the MS is attached to, i.e., only GPRS or both GPRS and other GSM services, and upon the MS’s capabilities to operate GPRS and other GSM services simultaneously.

– Class‑A mode of operation: The MS is attached to both GPRS and other GSM services, and the MS supports simultaneous operation of GPRS and other GSM services.

– Class‑B mode of operation: The MS is attached to both GPRS and other GSM services, but the MS can only operate one set of services at a time.

– Class‑C mode of operation: The MS is exclusively attached to GPRS services.

The three modes of operation are defined in GSM 02.60.

NOTE: Other GSM technical specifications may refer to the MS modes of operation as GPRS class‑A MS, GPRS class‑B MS, and GPRS class‑C MS.

5.4.6 Charging Gateway Functionality

The Charging Gateway Functionality (CGF) is described in GSM 12.15 [28].